Machine glazed paper



2,999,787 MACHINE GLAZED PAPER Martin L. Downs and Ralph A. Nelson,Appleton, Wis.,

assignors to Thilmany Pulp & Paper Company, a corporation of WisconsinNo Drawing. Filed Oct. 4, 1957, Ser. No. 688,142 7 Claims. (Cl. 1621'35)Unleveled base sheet formed (apparent density approx. 11 lbsJream/mil)Leveled Dried to substantially normal sheet dryness Hydrophilic colloidprepared which provides transparent film (water holding properties 3see.)

Colloid set on leveled surface of sheet without substantial penetrationColloid on sheet dried against smooth dryer surface to provide MachineGlazed Paper In the paper art, it has been known for many years that ifa wet cellulosic web is forced with suflicient pressure against a fiatsurface and the web is allowed to dry in contact with the surface, asheet will be provided having a side which conforms to the surface andwhich tends to reproduce the surface, i.e., a highly glazed surface.

In commercial paper making practice, it has been the practice to providesuch a highly glazed surface on one side of a paper sheet by pressing aweb onto the surface of a highly polished cast iron cylinder or dryerand hold the sheet in contact with the dryer until the moisture issubstantially removed from the web. When sufficient moisture has beenremoved, the sheet automatically releases itself from the dryer and hasa glazed surface which approximates the dryer surface.

The equipment used in the paper making art to produce such a highlyglazed paper is known as a Yankee machine or Yankee dryer or issometimes referred to as a Flying Dutchman machine. The papermanufactured on machines of this type is commonly known in the art asmachine glazed or M.G. paper.

As above indicated, the machine glazed paper obtains its glazecharacteristics by pressing of the cellulosic fibers, under highpressure, against a smooth polished dryer surface. The fibers on thesurface, substantially individually, are conformed to the smoothpolished surface. In order to obtain an improved glaze, the stock forthe Web of Patented Sept. 12, 1961 paper is beaten, or refined. However,such beating cannot be carried too far, i.e. to a low freeness levelsometimes referred to as a slow stock. If the porosity of the web isdecreased by using such a slow stock, thereby providing a dense sheet,vapor pressure builds up between the dryer surface and the web so thatthe web, in certain areas during drying, is prematurely lifted away fromthe dryer surface, thereby providing an inferior glaze in theprematurely lifted areas resulting in a checked or non-uniformly glazedsheet. Accordingly, the porosity of the sheet must be maintained to acertain degree, so as to prevent this premature lifting. In view ofthis, the extent of development of glaze has been limited.

Furthermore, it has been recognized in the paper art that the Web andresulting sheet do not faithfully reproduce the profile of the dryersurface, not only because of the inherent fibrous nature of the web,with the concomitant impossibility of pressing the sheet with suflicientforce to adhere every portion of the sheet to the dryer surface, butalso because the sheet is a spongy mass with only portions of the fiberin actual contact with the dryer surface.

As will be seen from the foregoing, there are inherent limitations inthe manufacture of machine glazed paper which necessarily limit thedegree of glaze which can be developed on the sheet. In this connection,the development of vapor between the dryer and sheet is inherent in thedrying operation, there are mechanical limitations upon pressure whichcan be brought to bear in contacting the sheet with the dryer surface,and the nature of the web limits the glaze which can be developed.

A principal object of this invention is the provision of an improvedpaper of the machine glazed type. A still further object of theinvention is provision of an improved process for making a paper havinga high gloss thereon. A particular object of the invention is theprovision of a process for manufacturing a high gloss paper, the glossbeing substantially transparent to thereby show the fibers of thecellulosic portion of the sheet.

These and other objects of the invention are accomplished by theformation of a web of paper or base sheet to which is applied an aqueousunpigmented mixture which includes a hydrophilic colloid. The colloid isset on the surface of the base sheet without substantial penetration ofthe base sheet, whereupon the combination is dried against a polishedsurface.

In the practice of this invention, the base sheet may comprise any fiberfurnish conventionally used in machine glazed papers. The surface shouldbe levelled prior to application of the colloid. Such levelling permitsthe establishment of high glaze with relatively low amounts of colloid.The wire side of the base sheet, i.e., the side of the sheet which rideson or is carried by the Fourdrinier wire of the paper making machine, islevelled prior to coating with the colloid. Levelling may beaccomplished by auxiliary wet pressing, a smoothing press, abreaker-stack, or by drying against a Yankee dryer. However, thelevelling process should not increase the apparent density of the sheetto more than about 11 pounds per ream (24 inches by 36 inches500 sheets)per mil of thickness.

The basis weight of the base sheet should be in the range of from 15pounds to pounds per ream (22 inches by 34 inches-500 sheets) finishedweight, including the usual moisture content. This weight range of basesheet is necessary to provide the sheet of the invention.

The base sheet, when the colloid is applied, comprises less than 60percent moisture. Higher moisture levels prevent the establishment ofthe high glaze surface because of difficulties in drying the sheet withthe colloid on the surface. To establish this moisture level, someinitial drying of the base sheet should be done to provide a base sheethaving the desired moisture level. The base sheet preferably has lessthan 40 percent moisture, and may be dried to normal sheetdryness. i Toprovide satisfactory glazing of the sheet, the colloid should provide onthe final sheet a continuous film over the surface of the base sheet.The establishment of such a continuous film can be determined by theuniformityof gloss and lac-k of imperfections in the finished sheet. .Inthis connection, the laboratory and semi-commercial work has shown that,with a 35 pound per ream base sheet, which has been levelled, as littleas 2.5 pounds of dried colloid solids per ream are required to produce ahigh and uniform glaze, but as much as 8 pounds of colloid per ream hasbeen successfully applied. In commercial practice with an air knife, agood level base sheet requires colloid to the extent of about 7 percentof the base sheet weight with a 35 pound per ream base sheet. With basesheets having lower basis weights, increased percentages of colloid arerequired. In this connection, a base sheet weighing 15 pounds per reamrequires colloid to the extent of 10 percent of the weight of the basesheet. The amount even, high gloss. Amounts in excess of a minimumamount can be applied but there is an increase in the dry- The selectionof a suitable colloid for appllcation to I the levelled sheet is ofgreat importance. Likewise, the

adjustment of conditions to assure that the resulting dried colloid is acontinuous film on the surface of the base sheet is of substantialimportance. Not only must a continuous film be provided but, inaddition, thefilrn should mirror the polished surface it is driedagainst. In

order to provide the desired film surface and film continuity, the filmshould be primarily on the surface .and

.of colloid will be kept as low as practical to insure the should notmigrate into the base sheet. If the colloid mi- 3 grates into the basesheet, a condition known as strike-in occurs. The condition of strike-inis readily disconcerned as an area of different appearance. Themigration can result from improper choice of colloid and/oncomditions ofmanufacture.

The overall spectral reflectance of the film carrying sheet is referredto as brightness, and strike-in results in loss of spectral reflectancewith consequent loss of brightness as measured by a brightness tester,which may be a Photovolt instrument. By way of example, a white sheethaving a basis weight'of 37.5 pounds per ream,

,and carrying a colloid film gave brightness readings of 67.5 to 68 onthe brightness tester in areas not showing strike-in while readings 65to 66 were obtained in areas showing strike-in. On a pastel yellow sheethaving a basis weight of 38 pounds per ream and having the colloid filmof the invention on its surface gave a brightness reading of 35 .5 to 36in areas free of strike-in and 33.5

to 34.5 in areas of strike-in. One can detect a one point 'loss inbrightness and rather easilyvisually detect .1.5

points loss in brightness.

The colloid materials used in the practice of this invention should havewater holding properties and should test higher than about 3 seconds. Inthe test, 10 grams of a colloid is prepared in 280 cubic centimeters ofdistilled water and held at 73 F. to 75 F. for 3 hours; Approximately 25cubic centimeters of the colloid mix is placed in a suitable porcelaindish. Straubel wet strength single fold kraft towels are cut into 1%inch squares andon'e side is dusted with a dry prepared mixture offinely ground corn starch, sugar and dye. The dusted sheet is droppedonto the colloid mix, dusted side up, and the time required applying thecolloid solution is important.

Dow Chemical Company--Methocel l0 cps. (methyl cellulose) Of the abovematerials, methyl cellulose is an excellent water holder and providesmost satisfactory results with minimum strike-in difficulties. However,those materials testing higher than 3 seconds can also providesatisfactory resutls.

To assure satisfactory filming, the colloid solution should have aviscosity of 200 to 1500 centipoises, as measured on a Brookfield 2MSpool at F. At lower viscosities, strike-in difficulties occur, while athigher viscosities, filming of the colloid solution onto the surface isdifiicult to accomplish.

It is desirable to have as high solids as possible in the colloidsolution providing that a viscosity in the range specified is provided.Ten percent solids is the lower practical limit for colloid solutions,while there are colloids available which can be used even though theycontain as high as about 35 percent solids.

The Penford 290 starch, at 20 percent solids in the colloid solution, isa very representative example of a suitable colloid mix. Protein andspecial gums such as Methocel have been successfully used but havepractical limitations because of cost, color, or viscosity.

In addition to having the foregoing properties, the colloid fiim whichis carried on the base sheet should have sufiicient flexibility so thatthe sheet can be embossed or printed. Furthermore, the flexibility ofthe colloid film should be such as to permit folding and bending of thesheet without cracking of the film. In addition, the colloid film shouldhave acceptance to inks so as to provide a good printing surface.

In order to prevent sticking of the film to the dryer surface, to whichthe film conforms, the colloid should release itself from the dryersurface upon drying of the combined sheet of the invention. Any stickingof the film to the dryer surface results in lessening of gloss of thesheet.

It will be understood, of course, that the colloid film can be treatedto develop a degree of insolubility in water so as to provide it withsubstantial resistance against retackifying upon redampening i.e.,becoming stick; on wetting. V

.The term apparent density used herein refers to the weight per ream(dry basis) divided by the calipered thickness of the sheet.

In preparing the sheet of this invention, the manner of As beforeindicated, a highly desirable way of applying the colloid solution is bymeans of an air knife. However, coating rollers and other systems may beemployed. It is important in the application of the colloid solutionthat the pressure be limited so as to reduce strike-in. Of course,

skill of the art to correlate these factors to provide the highly glazedsheet of the invention.

Not only. is it important to limit the pressure on the Lfil-m on thesheet in connection with the application of the jeo'lloid,but,,like'wise, it 'is important that the pressure .jonth'e' film belimited inthe early stages of drying on the face of the dryer. It willbe understood, of course, that as the degree of drying increases, i.e.,the sheet becomes dryer, greater pressures may be applied.

The conditions of pressure can best be adjudged by observation of theend sheet. In this connection, if insufficientpressure is being applied,flaked and skipped areas will appear on the face of the sheet of theinvention. On the other hand, if the pressure is too great, thestrike-in condition will appear, as above described.

Another important factor in the preparation of the sheet of theinvention is the temperature of the dryer surface which, in the case ofmost commercial operations, will comprise a Yankee dryer. Thetemperature of the surface of the dryer must be lou enough to preventblistering which causes loss of contact of a portion of the sheet withthe dryer surface because of the build up of vapor pressure. Thissurface temperature, of course, is proportional to the speed of thedryer, the amount of moisture present, and the weight and porosity ofthe base sheet. In addition, the blistering can occur as a function ofthe thickness of film on the base sheet. Here again, the temperature ofthe dryer surface should be adjusted, and adjustment is effected inoperation of the dryer to eliminate the blistering problem.

Blistering can be readily ascertained from inspection of the finishedsheet. By way of example, a sheet of this invention was dried on aYankee dryer having a temperature of about 190 F. to 220 F., the basesheet of the paper having a basis weight of 35 pounds per ream and thecontinuous film on this sheet weighing 3.5 pounds per ream.

It is also important that the apparent density be less than about 11pounds per ream per mil thickness. At higher densities, the porosity isso reduced that the sheet cannot be satisfactorily made. Accordingly, itis difiicult to obtain the desired levelling with presses and breakerstacks. Likewise, levelling cannot be satisfactorily obtained by coatingthe base sheet.

To provide commercial results, the base sheet should be glossy. In thisconnection, the base sheet should have a gloss of points.

As an example of the practice of this invention, a base sheet isprepared on a Fourdrinier section of a Yankee machine, the base sheethaving a basis weight of 35 pounds per ream, dry basis, and an apparentdensity of 10.5 pounds per ream per mil thickness. The sheet, in theFourdrinier machine, is dried to a moisture content of about 35 percent.The base sheet is levelled in a pressing section in the Yankee machine.

A colloid solution is prepared from a Penford 290 starch, the solutioncomprising 20 percent solids. No pigments or other opaque substances areadded to the colloid solution. The solution has a viscosity of 400centipoises. The colloid solution is applied to the base sheet by meansof an air knife to establish solids on one side of the base sheet at alevel of 3.5 pounds per ream. The base sheet, carrying the colloidmaterial, is passed into contact with a Yankee dryer, the colloidmaterial contacting the dryer. The surface temperature of the dryer isabout 190 F. When the sheet is dried, it releases from the dryer withoutdifficulty and the sheet is prepared for sale.

By way of another example, to a finished M.G. paper having an apparentdensity of 10.5 pounds per ream per mil thickness is applied the abovedescribed colloid solution. The M.G. paper carrying the film is dried ona polished Yankee dryer, as above described. Through the application ofthe colloid film, the gloss of the paper increases from about 25 pointsto about 65 points.

The base sheet may be sized or other wise treated in accordance withknown paper making procedures. Such sizing aids in limiting strike-in.Of course, the base sheet may be made from bleached pulp or unbleachedpulp. The base sheet may be colored, if desired.

The colloid film may likewise be colored but, as before indicated, itshould not be pigmented. Likewise, other inorganic fillers or materialsshould not be utilized in the colloid solution which materials break orinterfere with the filming characteristics of the colloid. The colloidshould provide a glaze on the sheet and, therefore, various of theelastomers cannot be used because they cannot be satisfactorily handledon the drier surface to provide the desired glaze.

It will be seen from the foregoing that a highly improved paper productcan be prepared by applying a colloid film to a levelled sheet of paper.The prior levelling of the sheet permits the establishment of unusuallyhigh glaze on the sheet. This levelling coacts with the colloid film,Which is carried on the surface of the sheet, to provide this highdegree of glaze when dried in contact with a polished dryer surface.

The various features of this invention which are believed to be new areset forth in the following claims.

We claim:

1. A process for the manufacture of machine glazed type paper comprisingthe steps of forming a base sheet having an apparent density of lessthan about 11 pounds per ream per mil of thickness, said base sheetbeing leveled and having a glossy surface, said base sheet being driedto substantial normal sheet dryness to thereby maintain the leveledsurface, preparing an aqueous mixture including a hydrophilic colloidwhich mixture provides a transparent film, said hydrophilic colloidmixture having water holding properties higher than about 3 seconds,setting said colloid on the said glossy surface of said base sheetwithout substantial penetration of said base sheet, and drying thecombined sheet with the colloid contacting a smooth drier surfacewhereby machine glazed paper having improved gloss is provided.

2. A process for the manufacture of machine glazed type paper comprisingthe steps of forming a base sheet having a basis weight of between about15 pounds per ream and about pounds per ream and having an apparentdensity of less than about 11 pounds per ream per mil of thickness,levelling at least one side of said base sheet to provide a glossysurface, said base sheet being dried to substantial normal sheet drynessto thereby maintain the leveled surface, preparing an aqueous mixtureincluding an hydrophilic colloid which mixture provides a transparentfilm, setting said colloid on the levelled surface of said base sheetwithout substantial penetration of said base sheet, said hydrophiliccolloid mixture having water holding properties higher than about 3seconds, and drying the combined sheet with the colloid contacting asmooth drier surface whereby machine glazed paper having improved glossis provided.

3. A process for the manufacture of machine glazed type paper comprisingthe steps of forming a base sheet having a basis weight of between about15 pounds per ream and about 120 pounds per ream and having an apparentdensity of less than about 11 pounds per ream per mil of thickness,levelling at least one side of said base sheet, said base sheet beingdried to substantial normal sheet dryness to thereby maintain theleveled surface, preparing an aqueous mixture including a hydrophiliccolloid which mixture provides a transparent film, said aqueous mixturehaving a viscosity of between about 200 centipoises and 1500centipoises, setting said colloid on the levelled surface of said basesheet without substantial penetration of said base sheet, saidhydrophilic colloid mixture having water holding properties higher thanabout 3 seconds, and drying the combined sheet With the colloidcontacting a smooth dryer surface whereby machine glazed paper havingimproved gloss is provided.

4. A process for the manufacture of machine glazed type paper comprisingthe steps of forming a base sheet having a basis Weight of between about15 pounds per ream and about 120 pounds per ream and having an apparentdensity of less than about 1 1 pounds per ream per mil of thickness,levelling at least one side of said base sheet, drying the base sheet tofinished sheet dryness '7 to thereby maintain the leveled surface,preparing an aqueous mixture including a hydrophilic colloid whichmixture provides a transparent film, said aqueous mixture having aviscosity of between about 200 centipoises and 1500 centipoises, settingsaid colloid on the levelled surface of said base sheet withoutsubstantial penetration of said base sheet, said hydrophilic colloidmixture having water holding properties higher than about 3 seconds, anddrying the combined sheet with the colloid contacting a smooth dryersurf-ace whereby machine glazed paper having improved gloss is provided.

5. A process for the manufacture of machine glazed type paper comprisingthe steps of forming a base sheet having a basis weight of between about15 pounds per ream and about 120 pounds per ream and having an apparentdensity of less than about 11 pounds per ream per mil of thickness,levelling at least one side of said base sheet, said base sheet beingdried to substantial normal sheet dryness to thereby maintain theleveled surface, preparing an aqueous pigment-free mixture including ahydrophi-lic colloid which mixture provides a transparent film, saidbase sheet having glossy surface, said aqueous mixture having aviscosity of between about 200 centipoises and 1500 centipoises, settingsaid colloid on the levelled surface of said base sheet withoutsubstantial penetration of said base sheet, said hydrophilic colloidmixture having water holding properties higher than about 3 seconds, anddrying the combined sheet with the colloid contacting a smooth dryersurface whereby machine glazed paper with improved gloss is provided.

6. A process for the manufacture of machine glazed type paper comprisingthe steps of forming a base sheet having a basis weight of between about15 pounds per ream and about 120 pounds per ream and having an apparentdensity of less than about 11 pounds per ream per mil of thickness,levelling one side of said base sheet, said base sheet being dried tosubstantial normal sheet dryness to thereby maintain the leveledsurface, preparing an aqueous pigment-free mixture including ahydrophilic colloid which mixturerprovides a. transparent film, saidaqueous mixture having a viscosity of between about 200 centipoises and1500 centipoises, setting said colloid on the levelledsurface of saidbase sheet without substantial penetrationoi said base sheet, saidhydrophilic colloid mixture having water holding properties higher thanabout 3 seconds, and drying the combined sheet with the colloidcontacting a smooth dryer surface whereby machine glazed paper havingimproved gloss is provided.

7. A process for the manufacture of machine glazed type paper comprisingthe steps of forming a base sheet having a basis weight of between about15 pounds per team and about pounds per ream and having an apparentdensity of less than about 11 pounds per ream per mil of thickness,levelling at least one side of said base sheet to provide a gloss of atleast 10 points, said base sheet being dried to substantial normal sheetdryness to thereby maintain the leveled surface, preparing an aque: ouspigment-free mixture including a hydrophilic colloid which mixtureprovides a transparent film, said aqueous mixture having a viscosity ofbetween about 200 centipoises and 1500 centipoises, said hydrophiliccolloid haviug a water holding capacity of at least about three secondsand comprising carboxy methyl cellulose, setting said colloid on thelevelled surface of said base sheet without substantial penetration ofsaid base sheet, and drying the combined sheet with the colloidcontacting a smooth dryer surface, whereby machine glazed paper havingan improved gloss is provided.

References Cited in the file of this patent UNITED STATES PATENTS1,753,565 Frank Apr. 8, 1930 1,992,996 Dodge Mar. 5, 1935 2,184,312Merrill Dec. 26, 1939 2,227,494 Gold Jan. 7, 1941 2,313,491 Adrian Mar.9, 1943 2,633,430 Kellgren Mar. 31, 1953 2,635,972 Azorloza et a1. Apr.21, 1953 2,656,286 Fisher Oct. 20, 1953 2,711,156 Bauling June 21, 19552,930,106 Wrotnowski Mar. 29, 1960 FOREIGN PATENTS 297,098 Great BritainAug. 8, 1929

1. A PROCESS FOR THE MANUFACTURE OF MACHINE GLAZED TYPE PAPER COMPRISINGTHE STEPS OF FORMING A BASE SHEET HAVING AN APPARENT DENSITY OF LESSTHAN ABOUT 11 POUNDS PER REAM PER MIL OF THICKNESS, SAID BASE SHEETBEING LEVELED AND HAVING A GLOSSY SURFACE, SAID BASE SHEET BEING DRIEDTO SUBSTANTIAL NORMAL SHEET DRYNESS TO THEREBY MAINTAIN THE LEVELEDSURFACE, PREPARING AN AQUEOUS MIXTURE INCLUDING A HYDROPHILIC COOLOIDWHICH MIXTURE PROVIDES A TRANSPARENT FILM, SAID HYDROPHILLIC COLLOIDMIXTURE HAVING WATER HOLDING PROPERTIES HIGHER THAN ABOUT 3 SECONDS,SETTING SAID COLLOID ON THE SAID GLOSSY SURFACE OF SAID BASE SHEETWITHOUT SUBSTANTIAL PENETRATION OF SAID BASE SHEET, AND DRYING THECOMBINED SHEET WITH THE COLLOID CONTACTING A SMOOTH DRIER SURFACEWHEREBY MACHINE GLAZED PAPER HAVING IMPROVED GLOSS IS PROVIDED.